1. Field of the Invention
The present invention relates to a charged beam drawing technique for drawing a fine pattern such as an LSI on a sample and, more particularly, to a displacement error measuring method which uses a charged beam to measure vibrations(relative displacement errors) occurring between the charged beam and a sample when the charged beam and the sample are moved, a charged beam drawing apparatus having a displacement error measuring function, and a semiconductor device which has a displacement error measuring mark and allows drawing of a desired pattern by compensating for relative displacement errors.
2. Description of the Related Art
Recently, as the degree of miniaturization and the packing density of semiconductor devices have been increased more and more, patterns to be formed on photoresist films also have been required to be finer patterns with a higher accuracy. Especially when such fine, high-accuracy patterns are necessary in, e.g., VLSIs, no desired patterns can be obtained by conventional methods of exposing patterns onto photoresist films by radiating light, since there is a limitation resulting from the wavelength of the light or the like. One method of solving this problem is to use an electron beam instead of light.
In an electron beam drawing apparatus, it is a common practice to draw a desired pattern on a sample by deflecting an electron beam while continuously moving a stage on which the sample is placed in one direction. In an electron beam drawing apparatus using this drawing method by continuous stage movement, vibrations of an X-Y stage have a large influence on the drawing accuracy. Therefore, the position of the X-Y stage is constantly measured by a laser interferometer, and, when the stage moves to a desired position, patterning is performed by sequentially shot-exposing a sample to an electron beam.
The development of an electron beam drawing apparatus of another type has advanced, in which the throughput is further increased by continuously moving not only a stage but a shaping mask for shaping an electron beam. That is, this electron beam drawing apparatus is a system for reducing and transfer-exposing a mask pattern onto the surface of a sample by using an electron beam, and comprises an aperture for shaping an electron beam emitted from an electron gun into a desired shape, means for radiating the charged beam shaped by the aperture onto a mask, means for reducing and projecting a region of the mask irradiated with the beam onto the surface of a sample, a mask stage for holding and moving the mask in a direction perpendicular to the axis of the beam, and a sample stage for holding and moving the sample in the direction perpendicular to the axis of the beam in synchronism with the mask stage. In this apparatus, the mask pattern is sequentially transferred to the surface of the sample.
An apparatus of the above kind, however, has the following problem. That is, if a relative vibration (either electrical or mechanical) that cannot be measured by a laser interferometer exists between the electron beam and the stage on which a sample is placed, this relative vibration cannot be compensated for. Consequently, the drawing accuracy becomes poor to make normal drawing impossible.
Such an unnecessary relative vibration can be measured by radiating an electron beam onto an edge of a fine mark which is placed on the stage and consists of, e.g., gold, and detecting reflected electrons produced upon the irradiation by using a detector. Furthermore, by reading the frequency of a vibration peak by analyzing the frequency of the resulting signal, it is possible to locate a portion posing a problem to some extent from the read frequency. Therefore, a certain counter-measure can be taken against the problem. This countermeasure, however, is limited to relative vibrations occurring while the stage is at rest, so no countermeasure can be taken against relative vibrations taking place while the stage is moved continuously. This is so because relative vibrations cannot be measured when the stage is moved.
FIGS. 1A to 1E are schematic views for explaining a conventional mark position detecting method. As shown in FIGS. 1A to 1E, a dot mark (Au mark) 27 consisting of a heavy metal such as gold is placed on the surface of a sample, and an electron beam (rectangular beam) 28 shaped into a rectangle is radiated on this mark. Since the amount of reflected electrons or secondary electrons produced changes in accordance with the overlap of the electron beam 28 and the mark 27, the signal amount detected by a detector changes accordingly. As shown in FIGS. 1A to 1D, during a period from the timing at which the electron beam 28 and the gold mark 27 begin overlapping each other to the timing at which they overlap completely, a certain relationship is established between the moving amount of the electron beam 28 and the detected signal amount in accordance with the shapes of the mark 27 and the electron beam 28. Therefore, the moving amount of the electron beam 28 can be determined from the detected signal amount. When the electron beam 28 is fixed to overlap the mark 27, a detection signal free from variations can be obtained ideally. If a relative vibration occurs between the electron beam 28 and the gold mark 27, the overlap of the beam and the mark changes, and this results in a change in the amount of reflected electrons. This makes it possible to measure the relative vibration between the electron beam 28 and the sample. In the above conventional method, however, this measurement cannot be performed while the sample is moved (while drawing is done) since it is impossible to keep radiating the electron beam 28 onto the gold mark 27 in that state.